Backscattering antenna array



Nov. 19, 1963 J w. CARR BACKSCATTERING ANTENNA ARRAY 5 Sheets-Sheet 1 Filed Oct. 26, 1960 H H H JALI 0 FIG 2 INVENTOR. JOHN W. CARR Agent Nov. 19, 1963 J. w. CARR 3,111,672

BACKSCATTERING ANTENNA ARRAY Filed 001.. 26, 1960 5 Sheets-Sheet 2 E (6) I800 megucycles 9 I60 2200 meqocycles E (9) 2900 megocycles FIG? O IN VEN TOR.

JOHN W. CARR 3 Agent Nov. 19, 1963 J. w. CARR 3,111,672

BACKSCATTERING ANTENNA ARRAY Filed 001,. 26, 1960 5 Sheets-Sheet 3 E (9) I800 megucycles of 9 l60 E (9) 3150 megocycles E (6) 2240 mequcycles FIG. l2

INVENTOR. JOHN W- CARR Agent Nov. 19, 1963 J. w. CARR 3,111,672

BACKSCATTERING ANTENNA ARRAY Filed Oct. 26, 1960 5 Sheets-Sheet 5 6 I00 W I IE! I 1 l l l I BK! l W M FIG. ll

INVENTOR. JOHN W. CARR Agent United States atet 3,111,672. BACKSCATTERKNG ANTENNA ARRAY John W. Carr, Santa Clara, Calif., assignor to Leckheed Aircraft Corporation, Burbank, Calif. Filed Oct. 26, 1960, Ser. No. 65,157 2 Claims. Cl. 343-753) This invention relates generally to antenna arrays for electromagnetic radiation and reception, and more particularly to a new form of antenna array which derives its directive characteristics from the relative phase and amplitude control of the backscattering from a periodic array of diffraction elements.

Accordingly, it is the broad object of this invention to provide a new type of antenna array operating on backscattering principles.

Another object of the invention is to provide a backscattering antenna array which is capable of producing a relatively narrow beam in a predetermined direction.

Still another object of this invention is to provide an antenna system operating on backscattering principles which permits the antenna to be designed to provide a predetermined radiation pattern.

Yet another object of this invention is to provide a backscattering antenna array which requires only a single feed point to excite the entire array without the need for transmission lines to distribute energy to remote elements of the array.

A further object of the invention is to provide an antenna array whose rate of change of broadside beam position with changes in frequency is relatively small.

A still further object of this invention is to provide a backscattering antenna array in accordance with the aforementioned objects which is low in height above the ground plane, is structurally simple, and can be constructed at relatively low cost.

The specific nature of the invention as well as other objects, uses and advantages thereof will clearly appear from the following description and the accompanying drawing in which:

FIGS. 1 and 2 are respectively top and front views of a backscattering antenna array in accordance with the invention.

FIG. 3 is a fragmentary view of FIG. 2.

FIG. 4 is a fragmentary view showing a modification of FIG. 2.

FIGS. 5- 8 illustrate E() radiation patterns obtained at various frequencies for a specific embodiment of FIGS. 1 and 2.

FIG. 9 illustrates an E() radiation pattern corresponding to the E( 0) pattern of FIG. taken at the angle 0 of maximum radiation thereof.

FIG. 10 is a graph of the angle 6 of maximum 0 plane radiation vs. frequency for four conditions of the diifraction elements of an antenna array in accordance with the invention.

FIG. 11 is a diagrammatic View of a balanced backscattering antenna array in accordance with the invention.

FIG. 12 illustrates an E(0) radiation pattern of the balanced antenna array of FIG. 11.

FIG. 13 illustrates another embodiment of a backseattering antenna array in accordance with the invention.

Like numerals designate like elements throughout the figures of the drawing.

In FIGS. 1 and 2, a backscattering antenna array is shown comprising a plurality of equal spaced diffraction elements '10 mounted perpendicularly to a ground plane 15. The array may be fed by means of a waveguide feed structure having dimensions W and S chosen to provide impedance matching and to permit only a single low order mode to be propagated. An arm 25a of the feed structure 25 couples the wave to the ditfracting elements 10 and its height H is ordinarily made on the order of one-half wavelength over the operating frequency range of the array.

Other feed structures may also be employed besides the structure 25 shown in FIG. 1, such as the use of a shorted transmission line stub in shunt formed by a strip or rod above the ground plane. The important requirement is that some feed means be provided to couple a wave to the array of diffraction elements 10. As is well known, an antenna may serve either as a radiator or as a receiver of electromagnetic energy with the same characteristics. In the use of the antenna array as a receiver, therefore, it will be understood that the feed structure 25 then serves to collect the energy received by the array.

Those skilled in the art will note that the general appearance of the antenna array of FIG. 1 is similar to a corrugated surface wave structure already known in the art. However, in the present invention the chief departure from such a structure is that the distance 1 between diffraction elements 10 is made of the order of 2 ()1 representing wavelength), which is very much larger than the distance provided between elements of the known corrugated surface Wave structure.

The defiraction elements 10 shown in FIGS. 1 and 2 then serve as lumped reactance discontinuities in the forward illuminating wave, each element 10 acting to backscatter (reflect) a portion of the wave while the remaining portion continues on to the other elements of the array. 1 have discovered that the height of the elements it which determines the value of the lumped constant thereof, and the spacing l therebetween can be chosen so that the relative phases of the element to element backscattering gives a backward constructive interference which causes the backscattered radiation to become predominate over a given frequency range. The result is that at a given frequency, a backwardly directed main beam can be obtained at a predetermined angle 0 the 0 plane beam width being dependent upon the length L of the array. The transverse width W of the elements 10 is important only in the sense that it determines the width of the beam in the plane, which is perpendicular to the 9 plane, and the feed structure 25 must be suitably chosen in accordance therewith.

The radiation pattern of the antenna array of FIGS. 1 and 2 may now be theoretically derived as follows. In the fragmentary view of FIG. 3, the 0 plane radiation pattern E(0) may be represented by the following Fourier series, the time variation exp( 'wt) being understood:

where A =peak amplitude of the scattering from the nth element G (9) =scattering pattern of the nth element A G (6) =any direct radiation from sources not accounted for in the scattering patterns of the elements F =phase function of the direct radiation relative to the array phasing W 60 (U W=27rf (v =phase velocity along the structure in the 0:0

or 0:180 (X or X) direction (v ),,=phase velocity in 6 direction 7 l =lumped phase delay introduced by diffraction over ith element in the forward illuminating wave m =total number of elements in the array where k=21r/ is the free space propagation constant. Any net phase delay in the structure greater than that which can be accounted for by the free space propagation constant k will be ascribed to the summation of the lumped phase delay, 1/ introduced at each element it Consequently, in evaluating the total phase delay of the forward wave illuminating the nth element, B X is set equal tokX in the first exponential factor of Equation 1 and the sum of the lumped phase delays from the preceding n1 elements is added to this phase delay by the second exponential factor of Equation '1.

If the elements are identical (that is, h is equal for all elements 10) the second exponential in Equation 1 becomes exp[-j(n1) Also, if the spacing I between the elements ltl are equal, then X =X +(n1)l, where X is the spacing between the source and the first element 10. For these conditions Equation 1 reduces to From Equation 2 it can be shown that the angle 0 of maximum 0 plane radiation from the antenna array of FIGS. 1 and 2 at a particular frequency may be determined by the following equation:

Effectively, Equation 3 states that the angle 0,, of maximum radiation is that angle for which 1 plus 1 cos 0-,, (FIG. 3) is equal to A or some multiple thereof so that at some far field point P(0 in the direction 0 the backscattered radiation from the individual elements 10 (or 10) will all be in phase.

Considering Equation 3 it will be seen that if t is zero or small corresponding to a small value of element height 11, maximum radiation will be obtained at an angle 6 =180 when l t/2, and at an angle 0 :90 when l=)\, For angles between 0 =90 and 0 =18'0, therefore, with t considered zero I would vary from l: 2 to [:h. Where ,0 is not negligible, the value of I will have to be modified accordingly as indicated by Equation 3. It must be realized, of course, that the shadowing and diffractive effect of adjacent elements 1t will limit directivity in the 180 direction, while the null of G(n) due to the lack of backscattering of the elements in the 1r/ 2 direction parallel thereto will limit directivity in the 1r/2 direction. However, to enable directivity to be obtained in the 1r/2 it is possible to tilt the elements forward by a sufficient angle a as indicated in 'FIG. 4 to maintain a finite G (6) at 6:1r/2.

It will be recognized in connection with the operation of the backscattering array of FIGS. 1 and 2 that the height h of an element It) (or 19 in FIG. 4) determines the amount of backscattering it provides. In order to achieve predominate backscattering operation, therefore, the number and height h of the elements 10 must be chosen so that the major portion of the energy in the wave is backscattered before reaching the last element of the array. Thus, for very long arrays lower element heights should be used so that less of the total energy at each element goes into the backscattering and more is left in the forward wave to illuminate a longer structure. This is advantageous in the sense that for longer, narrower beam arrays where the control of 0 is more critical it is also more accurately defined, since the value of b,, of the elements will be close to zero and thus more accurately bounded. For shorter arrays the height of the elements it (or 1%) must necessarily be greater to obtain predominate backscattering operation. 'Ilhe bandwidth obtained, therefore, will be larger so that a greater variation in 4 can be tolerated.

In order to provide a clearer understanding of the invention, the results of measurements on a specific embodiment of FIGS. 1 and 2 will now be presented, but it is to be understood that the presentation of this specific embodiment and the measurements made thereon is provided merely for illustrative purposes and it is not to be considered as limiting the scope of the invention in any way.

Referring to the letter dimensions in FIGS. 1 and 2, the following values are employed in the specific embodiment thereof:

l=3 inches M =4 Ai inches W =6 inches H =2 /s inches VVZI]. inch h=1 inch W =2" inches inch S=2 /2 inches L=3=9 inches X Ai inches g= /2 inch FIGS. 57 illustrate 13(0) radiation patterns obtained at various frequencies for the specific embodiment of FIGS. 1 and 2 having the above-listed dimensions. it will be noted that as frequency increases (corresponding to an increase in l With respect to A), the angle 0 of maximum radiation becomes smaller, and will approach with further increases in frequency. FIG. 8 is an E(t,l/) radiation pattern (the p plane being perpendicular to the 0 plane) taken at the same frequency as the E(6) pattern :of FIG. 5 at the angle of maximum radiation 0 thereof.

PEG. 9 is an E(9) radiation pattern for an antenna array in accordance with the invention which is substantially similar to that employed in making the patterns of FIGS. 5-8, except that the diffraction elements 10' are tilted by a=20i5, as indicated in FIG. 4, to obtain a broadside beam at 6 =90.

To illustrate the eliect of the lumped phase delay 11,, introduced by the diffraction elements in an antenna array in accordance with the invention, a number of curves of the angle 0 of maximum 0 plane radiation vs. frequency are plotted in the graph of FIG. 10 for four conditions .of the diffraction elements. Except for the diffraction elements, the structures employed in obtaining these measurements are basically similar to the specific embodiment of the antenna array used in obtaining the patterns of FIGS. 5-8.

In FIG. 10, the solid line curve represent the curve calculated from Equation 2 with 11,, set equal to zero corresponding to a negligible height h-0 of the diffraction elements. The measured points indicated by X correspond to a diffraction element height h of 1 inch as in the array used in making the patterns of FIGS. 58, while the measured points 0 correspond to a diffraction element height h of 0.63 inch. The measured point on the other hand, correspond to the tilted diffraction elements of FIG. 4 having a height h of 1 inch and an angle on equal to 20i5.

From FIG. it will be seen that except for 0 in the vicinity of 180 where the shadowing and idiiiractive efi'ect (of adjacent elements and the feed structure limit the proper operation of the area, and in the regions of 0 near 1r/ 2 where the null in G (0) is approached, the correlation between the measured points and the theoretical curve is good. For small element height the measured points (such as those indicated by 0 for 11:0.63) lie very close to the ip =0 curve. However, as the element height becomes larger, the effective value of 51/ is increased causing a larger displacement of the measured points. Except for the ranges of 0 near 90 and 180, therefore, the elfect of increased element height 12 or it is to displace the points upward :from the theoretical curve as shown.

It will also be seen from FIG. 10 that the rate of change of 0 with frequency gets smaller with decreasing 0 as the broadside (90) condition is approached and larger with increasing 0 as the endfire (180) condition is approached. This is the reverse of the forward traveling wave case, where the rate of change of 0 with frequency is greatest for the broadside operation and minimum for endfire operation. This small broadside rate of change of 0 with frequency in the baclss-cattering array of the present invention is advantageous in that it then becomes possible to obtain broadside operation over a larger useable frequency range.

It is to be understood in connection with the present invention that the backscattering antenna array is not limited to the basic form shown in FIGS. 14 and the principles of backsoattering which I have found applicable can also be extended to other advantageous forms. For example, as diagrammatically illustrated in FIG. 11, a balanced 0 plane hackscattering antenna array could be provided in which the ground plane 15 is eliminated and a balanced feed structure 125' is employed to illuminate diffraction elements 100 having twice the height 2h. as in the array of FIGS. 1 and 2. Any suitable structural arrangement (not shown) of non-conductive material (such as wood or plastic) may be used to support the diagrammatically illustrated diffraction elements 1%. FIG. 12 illustrates a typical E(0) radiation pattern at a frequency of 12400 megacycles tor a balanced backseattering antenna array in accordance with FIG. 11 having 14 diffraction elements 100 of height 211:2 inches and a spacing of 1:3 inches therebetween.

Other forms of backscattering antenna arrays in accordance with the invention are also possible, such as may be obtained by employing difieren-t transverse contours for the elements in a given array for beam-shaping purposes. One advantageous form involves choosing the transverse contours of the diffraction elements so that they form arcs 115 in the transverse plane as diagrammatically illustrated in FIG. 13. A suitable feed structure for these elements 110 is generally indicated at 225.

It will be recognized that there may be some advantage in certain applications in employing backscattering antenna arrays having difiiraction elements of unequal spacings and heights for beam-shaping or related purposes and the present invention is intended to include such modifications. Also, it will be apparent to those skilled in the art that the diffraction elements or ground plane could have a variety of equivalent forms and if desired metal mesh could be used rather than solid metal at appropriate frequencies. It is intended that such meanings be included in the use of the terms metal elements, metal strip, diilfraction elements and metal ground plane. Also, the use of the term ground plane includes various known equivalents thereof.

Still further, it will be realized that the basic array of FIG. 13 could be extended to a circular array in which a f ed structure for feeding a relatively small angular section could be mechanically rotated to provide scanning in an azimuthal plane.

It will be apparent, therefore, that many variations and modifications may he made in the construction and arrangements described and shown herein without departing from the scope of this invention as defined in the appended claims.

I claim as my invention:

1. A backscattering antenna array comprising a metal ground plane, :a plurality of spaced apart metal elements mounted on said plane, means mounted on said ground plane 'for feeding electromagnetic energy to said spaced apart elements, each of said elements comprising an elongated metal strip with the plane defined thereby at an acute angle with respect to said ground plane and at an acute angle to the direction of propagation of electromagnetic energy from said means, the spacing between adjacent elements being about one-half to one wave length at the operating frequency, each of said elements backscatters a portion of the electromagnetic energy incident thereon and the summation Otf the backsca-ttering by said plurality of elements comprises the predominant energy radiated from said plurality of elements.

2. The antenna of claim 1 wherein the planes defined by said elements have substantially the same height and have substantially the same spacing wherein the height and spacing of said elements are chosen in accordance with said angle such that the Plane of polarization of predominant energy radiated is perpendicular to said ground plane.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A BACKSCATTERING ANTENNA ARRAY COMPRISING A METAL GROUND PLANE, A PLURALITY OF SPACED APART METAL ELEMENTS MOUNTED ON SAID PLANE, MEANS MOUNTED ON SAID GROUND PLANE FOR FEEDING ELECTROMAGNETIC ENERGY TO SAID SPACED APART ELEMENTS, EACH OF SAID ELEMENTS COMPRISING AN ELONGATED METAL STRIP WITH THE PLANE DEFINED THEREBY AT AN ACUTE ANGLE WITH RESPECT TO SAID GROUND PLANE AND AT AN ACUTE ANGLE TO THE DIRECTION OF PROPAGATION OF ELECTROMAGNETIC ENERGY FROM SAID MEANS, THE SPACING BETWEEN ADJACENT ELEMENTS BEING ABOUT ONE-HALF TO ONE WAVE LENGTH AT THE OPERATING FREQUENCY, EACH OF SAID ELEMENTS BACKSCATTERS A PORTION OF THE ELECTROMAGNETIC ENERGY INCIDENT THEREON AND THE SUMMATION OF THE BACKSCATTERING BY SAID PLURALITY OF ELEMENTS COMPRISES THE PREDOMINANT ENERGY RADIATED FROM SAID PLURALITY OF ELEMENTS. 